proulx 1985 aquacultural-engineering

7/28/2019 Proulx 1985 Aquacultural-Engineering

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Aquacu l tura l Eng ineer ing 4 (1985) 93-111

G r o w t h o f D aph n i a magn a o n U r b a n W a s t e w a t e r sT e r t i a r i l y T r e a t e d w i t h Scenedesmus s p .

D . P r o u l x a n d J . d e l a N o f i eD6partement de biologie and Centre de recherche en nutri t ion,Universit6 L aval, Qu ebe c, Canada G 1K 7P4

A B S T R A C TT h e p r o d u c t i v it y a n d c o m p o s i t io n o f Daphnia magna w er e m ea s u r ed f o rh i gh d en s i t y p o p u l a t i o n s g r o w n i n 5 - 6 tr e f l o w - t h r o u g h t a n ks s u p p l ied w i t ha ter t iary ef f luent . This ef f lue nt was urban second ary wa stewaters b io logi-cal ly t reated w i th in ten sive cul tures o f a mieroalga Scenedesmus sp .

Three da i l y f eed ing ra tes {575 , 1150 , 175 0 mg dry we igh t a lgae day -1t a n U l ) , o b t a i n ed b y a d j u s ti n g t h e f l o w o f a t er ti a ry e f f lu en t co n t a i n in g115 m g dry we igh t o f a lgae l it re -1 , a ll owed average prod uct ions o f 0 .9 ,1 . 6 a n d 2 . 6 g o f Daphnia (wet basis) l i tre -1 w e eU 1 w h i ch w er e p r o p o r -t i ona l t o t he ra tions. When the f l o w o f t er ti ary e f f l uen t was increased to40 l it res day -1 t an U 1 (ad libitum), t he average y i e M rose to 3 .6 g o fdaphn M s {w et basis ) l i tre a w eek -1. The m ax im um pop u la t ion reached4 8 0 0 0 ind iv idua ls l it re ~. The daphn id /a lgae energy convers ion ra tio was0 .4 . A l g a e w er e r em o ved b y Daphnia a t a m a x i m u m e f fi c i en cy o f 95 % .T h e c o m p o s i t i o n o f Daphnia b iomasses was 59 .5% pro te in , 9 .5% fa t a nd5 .6% carb ohydra te on a dry we igh t bas i s . N o de f ic i enc i es in meth ion ineand l i no len i c ac id were no ted .Res u l t s i nd i ca t ed the remarkab le po ten t ia l o f urban was t ewaters f o r t hep r o d u c t i o n o f p r o t e i n a n d t h e h ig h p r o d u c t i v i t y o f Daphnia magna.

I N T R O D U C T I O NI n i n t e n si v e a q u a c u l t u r e , f e e d i n g liv e o r g a n i s m s m a y i n c r ea s e t h eg r o w t h ( G r e e n a n d M e r r i c k , 1 9 8 0 ) a n d s u r v iv a l r a t e s o f fi n g e r li n g s f o rs e v e r a l s p e c i e s o f fi sh a n d c r u s t a c e a n s ( B a r d a c h e t a l . , 1 9 7 2 ) . T h ec l a d o c e r a n D a p h n i a m a g n a is p a r t i c u la r l y w e l l a d a p t e d t o f r e s h w a t e ra q u a c u l t u r e ( H u e t , 1 9 7 0 ; I v le v a , 1 9 7 3 ; D i n g es , 1 9 7 4 ) o n a c c o u n t p a r ti -

93Aquacu l tura l Eng ineer ing 01 44 -8 60 9 / 8 5 / $ 03 .30 - E l sev ie r Applied S ciencePublishers Ltd, England, 1985. Printed in Great Britain


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94 D. Proulx, J. de la Noiiec u la r ly o f i ts h i g h r e p r o d u c t i o n p o t e n t i a l ( M y r a n d a n d d e la N o i ie ,1 9 8 2 ), h ig h p r o t e i n c o n t e n t ( R i c h m a n , 1 9 5 8 ; B o g a t o v a e t a l . , 1 9 7 3 ) a n du b i q u i t y ( Iv le v a, 1 9 7 3 ) . S i n ce t h e n a t u r a l p r o d u c t i o n o f D a p h n i ar e v ea ls i t s e lf i n s u f f i c i e n t t o m e e t t h e n e e d s o f a q u a c u l t u r e , s e ve r alr e s e a rc h g r o u p s a t t e m p t t o m a x i m i s e t h is p r o d u c t i o n b y i n te n s iv ec u l t u r e .

F o o d is a n e s se n ti al e l e m e n t in t h e p r o d u c t i o n o f D a p h n i a . T h ec u l tu r e m e d i u m m u s t b e a b u n d a n t , i n e x p e n si v e a n d m e e t t h e n u tr i t i o n a lr e q u i r e m e n t s o f t h is c l a d o c e r a n ( D ' A g o s t i n o a n d P ro v a s o li , 1 9 7 0;H e b e r t , 1 9 7 8 ) . N u t r i t i v e s u b s t r a t e s s u c h a s b a c t e r i a ( T e z u k a , 1 9 7 1 ) ,y e a s t s ( N a n d y e t a l ., 1 9 7 7 ) a n d m a n u r e ( D e P a u w e t a l . , 1 9 8 0 ) a r e p o o rs u b s t i t u t e s f o r m i c r o a lg a e as t h e m a j o r f o o d s o u r c e o f D a p h n i a . H o w -e v e r , m i c ro a l g a e p ro d u c t i o n o n a l a rg e s c a l e w i t h f e r t i l i s e r s i s v e rye x p e n s iv e . O n t h e o t h e r h a n d , s e c o n d a r i ly tr e a t e d u r b a n w a s t e w a t e r s ,w i t h t h e i r f e r t i l i s i n g p o t e n t i a l ( N H ~ , N O ; , P O ~ - ) , a r e a n a d e q u a t e a n di n e x p en s i v e m e d i u m f o r t h e p r o d u c t i o n o f i m p o r t a n t a lg al b io m a s s es( S h e l e f a n d S o e d e r , 1 9 8 0 ) t h a t c a n b e u s e d b y D a p h n i a ( M y r a n d a n dd e l a N o i i e , 1 9 8 2 ) .

T h e s e c o n d m a j o r f a c t o r i n t h e p r o d u c t i o n o f D a p h n i a r e f e r s t o t h ec u l tu r e m e t h o d s a n d t h e p h y s i c o -c h e m i c a l c o n d i t i o n s o f th e g r o w t hm e d i u m . T h e s e c o n d i t i o n s w i l l a f f e c t g r o w t h ( I v l e v a , 1 9 7 3 ; L a m p e r t ,1 9 7 7 ; H e b e r t , 1 9 7 8 ) an d r e p r o d u c t i o n o f D a p h n i a ( F o x e t a l . , 1 9 5 1 ;G r e e n , 1 9 5 6 , 1 9 6 6 ; W i n n e r a n d F a r re l l, 1 9 7 6 ; L a m p e r t , 1 9 7 8 ) . A c c o r d -i n g t o I v le v a ( 1 9 7 3 ) , t h e s e p a r at e c u l t i v a t io n o f d a p h n i d s a n d t h e i r f o o do b j e c t s g iv e s b e t t e r r e s ul ts t h a n p r o d u c i n g b o t h s i m u l t a n e o u s ly . F u r t h e r -m o r e , S o r g e l o o s e t a l . ( 1 9 7 9 ) o b s e r v e d t h a t t h e p r o d u c t i v i t y o f A r t e m i ai n cr e as e s w h e n t h e s e c u l t u r e s a re s u b m i t t e d t o a c o n s t a n t le ve l o f f o o da n d t o a c o n t i n u o u s d i l u t i o n o f m e t a b o l i c w a s te .D a ta o n t h e p r o d u c t i o n o f D a p h n i a m a g n a o n t e r t i a r y e f f l u e n t s o fu r b a n o r i g i n a re s c a r c e ( D e W i t t a n d C a n d l a n d , 1 9 7 1 ; M y r a n d a n d d e laN o t i e , 1 9 8 2 ; Ta r i f e f io -S i l v a e t a l . , 1 9 8 2 ) a n d d o n o t c o n t a i n a n a ly s e so f t h e c o m p o s i t i o n o f t h e a n i m a l b i o m a s s e s p r o d u c e d . I t h a s b e e nd e m o n s t r a t e d ( S n o w , 1 9 72 ; B o g a to v a e t a l . , 1 9 7 3 ; L e m c k e a n dL a m p e r t , 1 9 7 5) t h a t d a p h n i d c o m p o s i t i o n m a y v ar y a c c o rd i n g t of e e d i n g a n d r e a r in g c o n d i t i o n s . F u r t h e r m o r e , v e r y f e w s t u d ie s ( L a m p e r t ,1 9 7 6 ; H e i m b u c h , 1 9 7 8 ; H e is ig , 1 9 7 9 ) d e a l w i t h t h e c o n t i n u o u s f e e d i n go f D a p h n i a . W i t h t h e a i m o f o b t a i n i n g u s e f u l d a t a f o r p l a n n i n g f u t u r es t r a t e g i e s t o g r o w D a p h n i a p o p u l a t i o n s , t h i s s t u d y w a s u n d e r t a k e n w i t ht w o g o a l s: 1. t o m e a su re t h e y i e l d o f D a p h n i a m a g n a w h e n s u b m i t t e d


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G r owi ng Daphnia rnagnaon wastewaters 95to various levels of microalgae (tertiary effluent) supplied in a flow-through system; 2. to measure the chemical composition of the bio-masses of D a p h n i a produced.

MATERIALS AND METHODSM i c r o a lg a e c u l t u r eThe secondary urban effluent is produced at the wastewater treatmentplant of Valcart ier's mili tary base (Qu6bec). Wastewater (NH,~, 990 gM;N/P -- 11) is collected at the exit of the secondary sedimentation basinbefore chlorination. The secondary effluent was submitted to a tertiarybiological treatment (Picard e t a l . , 1979; de la Nofie e t a l . , 1980) byintensively growing a strain of the green alga S c e n e d e s m u s sp., originat-ing from the secondary effluent itself.

Phytoplankton was grown in a semi-continuous system under artifi-cial light (14 light: 10 dark: 30 000 lux) and at a temperature of 20-+ IC.Algae were maintained in suspension by forced aeration without CO2supplementation. Every day at the same hour, 55% of the culturevolume was removed and replaced by the secondary effluent dilutedwith tap water (for practical purposes) to a final concentration of 65%.This renewal ensured the suspension of algae with a constant qualityand a constant biomass concentration (115 mg dry weight litre-l:- 1 106 cells ml-1). This tert iary-type eff luent was poured into anopaque container where algae were maintained suspended by agitationwith a magnetic bar.P r o d u c t io n o f Daphn iaD a p h n i a m a g n a originated from a population grown under the sameconditions o f illumination, temperature and feeding for several genera-tions. D a p h n i a were grown in 5-1itre rectangular containers, under low-intensity illumination (14 light: 10 dark: 300 lux), and at a tempera tureof 20 + IC. A peristaltic pump (Sage Inst ruments) continuously suppliedD a p h n i a with the tertiary effluent containing microalgae. The excessmedium flowed through a net (Nitex, 530/am) fixed up horizontallyinto the water to keep the D a p h n i a in the culture container.


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96 D. Pro ulx, J. de la No iieTrials for D a p h n i a production were performed with two groups:1. Populations fed the same daily ration of microalgae (575, 1150 or1725 mg dry weight day 1) during the whole exper iment; the

initial D a p h n i a population was 100 individuals litre -~ of 1-5 mmlength (caudal spine excluded).

2. Populations fed algae a d l i b i t u m by adjusting the flow (9 litres day-1up to 40 litres day-1) of ter tia ry ef fluent (115 mg dry weight of algaelitre-~) to population density; the initial population was 1000 indi-viduals litre-1. Experiments lasted one month and were performedin duplicate.

The net increase rate of the population (r') was determined by thefollowing equation (Winberg e t al . , 1971 ):

r' = (ln N t -- In N o ) / twhere No = population density (N litre -1) at time to , Nt = populationdensity (N litre -1) at time t.Sam pling and analysesAlgal biomass in the tertiary effluent and at the exit of D a p h n i a culturetanks was measured by filtering the samples on Whatman GF/C (1.2/am)filters which were then dried at 90C for 24 h. Population density andD a p h n i a biomass were estimated every 3 days by removing three 40-mlsamples. These samples were sieved through two nets (Nitex, 530 tamand 1400 tam) separating three categories of sizes (excluding the caudalappendix), i.e. juveniles ( 2-7 ram). The number of D a p h n i a carrying eggs was noted.Dry weight of D a p h n i a was determined by drying the samples at 90Cfor 24 h.

For protein, lipid, carbohydrate, energy and amino acid analyses,the samples of algae and D a p h n i a biomasses were lyophilised; the dataobtained are thus mean values. Proteins were analysed by a Kjel-Fossautoanalyser (model 16210) using a conversion coefficient of 6.25.Lipid content was measured after ether extraction with a Goldfishapparatus; characterisation of the lipid fractions was done by agas-liquid chromatograph (Hewlett Packard, 5790A series)equippedwith flame ionisation det ector and capillary fused silica column (DB225,0.25 mm internal diameter: 25 m long, liquid phase Durabond OV225,


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Growing Daphnia magna on wastewaters 970 . 2 5 / ~ m ) ; t h e c a r r i e r g a s w a s H2 ( s p l it t e r r a t e 1 / 40 0 ) ; t h e o v e n t e m p e r a -t u re w a s 6 0 - 1 9 0 C ( 10 C m i n - l ) ; i n j e c t o r t e m p e r a t u r e , 2 10 C ; d e t e c t o rt e m p e r a t u r e , 2 2 0 C ; c a r b o h y d r a t e s w e r e m e a s u r e d b y t h e m e t h o d o fK o c h e r t ( S t ei n , 1 9 7 3) ; c ru d e fi b re w a s e s t i m a t e d b y t h e A O A C m e t h o d( A O A C , 1 9 8 0 , a r t. 7 0 6 1 t o 7 0 6 5 ) ; m i n e r a l c o n t e n t w a s c a l c u l a t e d b yi n c i n e r a t i n g t h e s a m p l e s a t 5 5 0 C f o r 6 h ; e n e r g y c o n t e n t w a s m e a s u r e dw i t h a P a r r a d ia b a t i c c a l o r i m e t r i c b o m b ; a m i n o a c i ds w e r e d e t e r m i n e db y a T e c h n i c o n a u t o a n a l y s e r ( m o d e l T S M ) u s in g n o r l e u c i n e a s a ni n t e r n a l s t a n d a r d ( 2 . 5 t a m o l l i t r e - a ) .

R E S U L T SP o p u l a t i o n dynamicsF i g u re 1 s h o w s t h e e v o l u t io n o f p o p u l a t i o n d e n s i t y a s a f u n c t i o n o ft h e d a i l y r a t i o n o f a lg a e. T h e m a x i m u m p o p u l a t i o n d e n s i ti e s f o r t h ev a r io u s f i x e d r a t i o n s o f a l g ae ( 5 7 5 , 1 1 5 0 a n d 1 7 25 m g d r y w e i g h t o fa l g a e , d a y - 1) w e r e 1 2 0 0 0 , 2 1 0 0 0 a n d 4 8 0 0 0 i n d i v i d u a l s l i t r e - 1, r e s p e c -t iv e ly . T h e s e m a x i m a w e r e r e a c h e d a f t e r 2 2 , 2 8 a n d 3 3 d a y s o f c u l t u r ea n d w e r e i m m e d i a t e l y f o l lo w e d b y a de c r ea s e in p o p u l a t i o n d e n si ty .

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I I I i I f I I t0 4 8 1 2 1 6 2 0 2 4 2 8 3 2 3 6D A Y SFig. 1. Evolution in m ean pop ulation dens ity of D. magna as a func t ion o f foodsupply. (e) ad libitum, (c~) 1725, (v ) 1150 and (o) 575 mg dry weight a lgae day 1 .


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98 D. Proulx, J. de la Noiiea

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Fig. 2. Ev olution o f the size distribution of D. magna as a funct ion o f food supply.(a) 575, (b) 1150 an d (c) 1150 m g dry weight algae day-X; d) ad libitum. (A) Ovi-gerous adults; (B) adults; (C) pre-adults, (D) juveniles. (oe) Replicates.

M o r t a l i t y a f f e c t e d m o s t l y j u v e n i le s a n d w a s h i g h e r i n p o p u l a t i o n s f e da t t h e h i g h e s t r a t e .

I n c r e a s in g t h e m i c r o a l g a e r a t i o n d i d n o t a f f e c t t h e d u r a t i o n o f th e l a gp h a s e , w h i c h w a s 1 0 d a y s . T i m e l ag is a f u n c t i o n o f t h e si ze c o m p o s i t i o no f Daphnia p o p u l a t i o n s a t t h e b e g i n n i n g o f t h e e x p e r i m e n t . T h e m a x i -m u m d e n s i t ie s o f f e m a l e s c a r r y i n g e gg s f o r t h e t h r e e p o p u l a t i o n s r e ce iv -i n g a f i x e d a l ga e r a t i o n w e r e 1 2 5 0 , 1 5 0 0 a n d 2 0 0 0 i n d i v i d u a l s l it r e -1 ,r e sp e c ti ve l y . T h e s e m a x i m a w e r e o b t a i n e d a p p r o x i m a t e l y 2 d a y s b e f o r e


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Growing Daphnia rnagnaon wastewaters 99D a p h n i a populations reached their maximum density. In populationsfed algae a d l i b i t u m , the lag was only 7 days and the populationdensity, after 34 days of growth, was 35 000 individuals litre-1. The netincrease rate of population (r') was 0.2 for D a p h n i a populations fed thevarious fixed rations and O. 1 for those fed a d l i b i t u m .

The size distribution of D a p h n i a populations as a function of time ispresented in Fig. 2. Replicates showed identical variation patterns. Sizecategories were similar for populations fed fixed rations (Figs 2(a), (b)and (c)) but they differed from those of populations f e d a d l i b i t u m (Fig.2(d)). In the latter, mass arrival of juveniles after 14 days affected theother size categories.ProductivityFigure 3 shows the evolution of standing crop biomasses of D . m a g n aas a function of food supply. For the various fixed rations (575, 1150and 1725 mg dry weight of algae day-l), the maximum daphnid

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4 8 1 2 1 6 2 0 2 4 2 8 3 2 3 6D A Y S

Fig. 3. Evolution of the standing crop biomasses of D. magna as a function offood supply. (o) ad l ib i tum, (n) 1750, ( v ) 1150 and (o) 575 mg dry weight algaeday 1.


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100 D. Proulx, J. de la Noftebiomasses obtai ned were 280, 615 and 1025 mg dry weight litre-x,respectively, which corresponds to mean biomass increases of 192 ,2 98and 447 mg d ry weight litre -x week -x, respectively. Pre-adults and adultsaccounted for almost 70% of the maximum biomass obtained althoughthey only contributed 30% of the total populations. The average weightof D a p h n i a remained low during the sharp increase of populati ondensity and then increased with the decrease in population. Therelationship between maximum daphnid biomass (Bm) and daily algalration (RQ) is defined as follows:

B m = 3. 2R Q -- 100 (r 2 = 0.99)where B m = mg dry weight D a p h n i a litre -x and RQ = mg dry weigh talgae l itre -x of cult ure medium day -1.

The biomass of D a p h n i a populations fed algae a d l i b i t u m was 2040mg dry weight litre-x after 34 days of growth, which corresponds to amean biomass increase of 420 mg dry weight litre-x week-x. The energyconversion ratio was 0.4 and microalgae were removed by D a p h n i a witha maxim um efficiency of 95%.

T A B L E 1Compositions of Daphnia m agna, Scened esmus sp., Artemia salina and Grower

Pellets for SalmonidsCons t i tuen t Scenedesmus D. magnaa Daphnia b A. salinac G r o wer

(% dr y we ight) sp. pellets aCrude protein 44.3 59.5 17-60 58.0 40.0

(N 6-25)Crude fat 9.5 9-5 4-26-7 5.1 10.0Carbohydrate 29-2 8.7 1.1-33 - -Crude fibre 0.7 5.6 - 3.5 3.0Ash 7.9 15.5 16-33 20.6 -Energy content 21.2 20.0 - - 22.4

(kJ g-1 dry weight)a Data obtained with random samples of all categories of sizes.b Ivleva (1973).c Gallagher and Brown (1975).a Martin Feed Mills Ltd. (MNR, 82g).


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Growing Daphnia magnaon wastew aters 101C o m p o s i ti o n o f D. magna an d Scenedesmus sp.The data of Table 1 reveal that the compositions of D a p h n i a andS c e n e d e s m u s differed markedly. Protein, crude fibre and ash contentsof daphnid were higher than those observed for algae while carbohydrateand fat contents were, respectively, lower and equal. The energy valueof S c e n e d e s m u s sp. was slightly higher than that measured for D a p h n i a .

The amino acid profiles of S c e n e d e s m u s sp. and D . m a g n a are pre-sented in Table 2. The essential or semi-essential amino acid con ten t ofD a p h n i a was either equal to or higher than those o f microalgae exceptfor leucine, histidine and arginine which were lower. The fat composi-tion of D . m a g n a is presented in Table 3. Fatty acids were mainly 16and 18-carbons. Palmitic (C16), oleic (C18:1) and linolenic (C18:3 )acids appeared in large amounts.

DISCUSSIONFood quantity and quality as well as temperature of the medium aremajor factors affecting daphnid growth and reproduc tion (Hall, 1964:D'Agostino and Provasoli, 1970; Weglenska, 1971: Vijverberg, 1976:Kersting, 1978; Lei and Armitage, 1980; Myrand and de la Nofie, 1982).By increasing the daily microalgae supply, we have increased thethreshold of support capacity in the medium. We observed an elevationin the number o f ovigerous animals (Fig. 2) which resulted in an increaseof population density and of the standing crop biomass of daphnids inthe culture tanks (Figs 1-3). The maximum population densi ty observed(48 000 ind. litre -1) was high as compared to the data r eported in theliterature (10 000 ind. litre -1) (De Pauw e t a l , , 1981) for intensive culture(8-1itre tank) o f D a p h n i a m a g n a grown on rice bran.

Contrary to populations of P a r a m e c i u m and T r i b o l i u m (Gause,1935), D a p h n i a populations do not reach an asymptote as a function offood availability (Pratt, 1943). The behaviour of these populationsdepends on the time needed lbr adjusting reproduction following analteration of population density. For a fixed daily ration in algae,increasing population density leads to a decline in the amount of foodavailable per individual as well as an increase in the concentration ofmetabolic wastes in the medium (Frank e t a l . , 1957). Both these factorsresult in a sharp decrease in population density affecting most ly juveniles.


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102 D. Proulx , J . de la Noi ie

T A B L E 2Amino Acid Composition of S c e n e d e s m u s sp., D a p h n i a m a g n a , Brine Shrimp and

Amino Acid Requirements for O n c o r h y n c h u s t s h a w y ts c h a (g 100 g-~ protein)A m i n o a c i d Scenedesmus D. magna D. magnaa B r in e A m i n o a c i d

sp. shr imp b r equ i r e men t sforOncorhynchustshawytschac

Essent ia lLeu 8.3 8.0 8.3 8.0Ile 4.1 5.0 5-2 5.3Lys 5.9 7.0 10.1 7.6Thr 8.6 9.7 4-8 4.6Try n.m.y n.m. n.m. 1.0Val 5.7 5.9 6.1 5.4Met 2.4 2.8 1.1 2.7Phe 5.7 5.7 5.0 4.7His 1.9 1.4 4.6 1.8Arg 6-0 4.9 6.1 6-5Total a 42.9 44-7 46-3 41.9

Non-es sen t ia lAla 7.1 6-1 6.5 6.9Asp 10.5 8.8 6.8 9-2Cys 0.5 2.5 n.m. 2-2Glu 11.2 9-8 12.5 14.2Gly 6.4 5.3 6.3 5.3Pro 9.0 9.4 5.1 5.2Ser 2.3 3.1 5.6 4.8Tyr 4.4 5.6 4.5Total e 46.5 42.5 42.8 45-6

3.92.55-02.30.53.21.51.76.0

26.1

a Morales (1983). b Gallagher and Brown (1975). c Halver (1972) . a Minus Try and.Phe. e Minus Cys and Tyr. f n.m., not measured.


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Growing D ap h n i a mag n a on wastewaters 103T A B L E 3C h ar ac t e r i s a t i o n o f t h e L i p i d F r ac t i o n s

( Ex p r e s s ed a s P e r cen t ag e o f t h e To t a lF r a c t i o n ) o f Daphnia magna G r o w n o nScenedesmus s p . an d C u l t u r ed o n R i ce B r an

Fatty acid D. ma gna D. magnafed rice bran a

1 0 : 0 3 .8 -1 2 : 0 0 . 6 -1 4 : 0 6 .9 3 -914 :1 3 .4 4 .115 :0 2 .2 1 .01 5 : 1 - 0.71 6 : 0 2 5 .2 2 1 .216 :1 7 .7 18.21 6 : 2 2 .4 -1 6 : 3 2 .3 2 .21 7 : 0 3 .2 -n . i .b 3 .0 -n . i . 5 . 8 -1 8 : 0 1.8 3-318 : 1 9 .7 26.018 :2 5 .1 20 .018 :3 17 .8 0 .22 0 : 0 T r ace T r ace

a Mora l es (198 3) . b n . i ., no t i den t i f i ed .

T h e h i g h e r m o r t a l i t y r a t e o f j u v e n i l e s m a y b e e x p l a i n e d b y a d e p l e t i o no f th e i r f a t r e s e rv e . L e m c k e a n d L a m p e r t ( 1 9 7 5 ) o b s e r v e d t h a t d u r in gf o o d d e p l e t i o n , d a p h n i d s u se l ip id s a n d s u ga r s r a t h e r t h a n p r o t e i n s .B i r ge a n d J u d a y ( 1 9 2 2 ) h a v e s h o w n t h a t j u v e n i l e s c o n t a i n o n l y 4 % f a ta s c o m p a r e d t o 2 1 % f o r o v ig e r o u s a d u l t s i n n o r m a l f e e d i n g c o n d i t i o n s .F u r t h e r m o r e , T e s s i e r a n d G o u l d e n ( 1 9 8 2 ) a n d T e s si e r et al. ( 1 9 8 3 ) h a v es h o w n t h a t t h e n u m b e r o f f a t d r o p l e t s ( d i s t r i b u t e d t o th e e gg s b ym a t e r n a l o v a r i e s ) w h i c h a r e r e s p o n s i b l e f o r t h e r e s i s t a n c e o f j u v e n i l e st o s t a r v a t io n , i s a f u n c t i o n o f t h e a m o u n t o f f o o d a v a i la b le t o a d u l t s .

A s o b s e r v e d i n o u r e x p e r i m e n t s , v a r i a t i o n s in t h e p o p u l a t i o n w e r ea s s o c i a t e d w i t h c h a n g e s i n a ge a n d s iz e c o m p o s i t i o n ( F i g . 2 ) r e s u l t in g


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104 D. Proulx, J. de la Noiiefrom changes of the age-related specific survival and reproductive ratesoccurring when the conditions of the medium are altered, for instancewith a decrease of algae concen tra tion per individual. According toSlobodkin and Richman (1956), age structure and variation of physio-logy with age are of major importance in the growth of populations ofmetazoa. With De Pauw et al. (1981) and Ivleva (1973), we believe thatregular sampling of the population is necessary to maintain constantconditions, especially feeding conditions, and thus ensure a high growthand reproduction rate of Daphnia.

The direct relationship obtained between the daily microalgae rationand the maximal daphnid biomass did not permit us to predict the maxi-mum capacity of the medium to support daphnids, which dependspart ly on space and physico-chemical constraints (Dinges, 1973; Ivleva,1973). Ad libitum supply of microalgae led to an increase in produc-tivity (Fig. 3) but this method has limitations: excessive concentrationsof suspended algae result in a decrease in the digestion effic iency ofDaphnia and cause obstruction of the filtration apparatus which maylead to death (Ivleva, 1973). This might explain the lower productivityfor the population submitted to ad libitum feeding as compared to thatreceiving a daily algae rat ion of 1725 mg dry weight.Mean productivities recorded in our experiments are far higher thanresults published previously (Table 4). The best yields were obtained byIvleva (1973) and De Pauw et al. (1981) who used algae and rice bran,respectively. The production of daphnids by De Witt and Candland(1971) from algae grown on wastewaters was only 0.1 g wet weightlitre -1 week-1. The energy conversion ratio obtained in our experiments(0.4) was the same as that obtained by Tarifefio-Silva et al. (1982) forcultures (13-1itre tanks) of Daphnia magna fed with microalgae grownon wastewaters.

This increase in productivity can be explained by the flow-throughculture system and by the development of specialised microflora on thewalls of the culture tanks (unpublished results). Continuous supply ofalgae suspension allows the elimination of metabolic wastes and providesdaphnid populations with constant feeding conditions (Ivleva, 1973;Heimbuch, 1978), thus avoiding mass deposition of microalgae andsharp variations in the concentrations of microalgae that are harmfulto daphnids.On the other hand, recent studies have shown a close and beneficialrelationship between bacterial microflora and Daphnia. The work of


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G r o w i n g Daphnia magna on was t ewaters 105T A B L E 4Culture o f D a p h n ia on Various Nutri t ive Substrates

S u b s t ra t e s Pr o d u c t i v it y Re f e r en ces( g w e t w e i g h tl i tre- 1 we ek - l )

Rice bran 0-5- 0.6M icroalgae grown on pig manure 0.1Yeasts and fertiliser 0. 1- 0.2 8M icroalgae grown on wa stew aters 0-1Yeasts 0 .21-0.35Microalgae 0.5-0.7Microalgae 0.57Ferti l iser 0-01-0.06M icroalgae grown on wa stewa ters 3.6

De Pauw et al . (1 9 8 1 )De Pauw et al . (1 9 8 0 )Bogatova and Askerov (1965)De Witt and Candland (1971)Briskina (1960)lvleva (19 73 )Heimbuch (1978)Heisig (1979 )This wo rk

M y r a n d a n d d e l a N o f ie ( 1 9 8 2 ) h a s e v i d e n c e d a p o s i ti v e i n t e r a c t i o nb e t w e e n t h e m i c r o f l o r a e n d o g e n o u s t o t h e u r b a n s e c o n d a r y e f fl u e n ta n d t h e p r o d u c t i v i t y o f D a p h n i a m a g n a . T e z u k a ( 1 9 7 1 ) a n d C o v e n e ye t a l . ( 1 9 7 8 ) h a v e d e m o n s t r a t e d t h a t b a c t e r ia m a y a c c o u n t f o r a f ai rp a r t o f t h e d i e t o f D a p h n i a a n d t h a t t h e y m a y s t im u l a t e t h e g r o w t h o fj u v e n i l e s . M o r e o v e r , H a d a s e t a l . ( 1 9 8 3 ) h a v e s h o w n a s y m b i o t i c r e l a -t i o n b a c t e r i a - d a p h n i d i n t h e d i g e s t i o n o f E s c h e r i c h i a c o l i ce l l s .

D a t a o n t h e m e a n c o m p o s i t i o n o f D a p h n i a m a g n a i n t h i s s t u d y r a n g eb e t w e e n t h e e x t r e m e v a l u e s ( Iv le v a , 1 9 7 3 ) r e p o r t e d i n t h e l it e r a tu r e( T a b l e 1). P r o t e i n c o n t e n t w a s a m o n g t h e h i g h e s t v a l u e s , t h e o t h e rc o n s t i t u e n t s b e i n g m o s t l y a t t h e l o w e r li m i t. V a r i a t i o n s in t h e c o m p o s i -t i o n r e p o r t e d in t h e l i t e r a t u r e r e s u l t e d t o m f o u r m a j o r f a c t o r s , i. e.s ta g e o f d e v e l o p m e n t o f D a p h n i a ( R i c h m a n , 1 9 5 8 : B o g a t o v a e t a l . ,1 9 7 3 : I v le v a , 1 9 7 3 ) , f o o d q u a l i t y ( M o r a l e s, 1 9 8 3 ) , f o o d a v a il a b il i t y( B o g a t o v a e t a l . , 1 9 7 3 ; L e m c k e a n d L a m p e r t , 1 9 7 5 : T e s si er e t a l . ,1 9 8 3 ) a n d f i na lly , t e m p e r a t u r e o f th e c u l tu r e m e d i u m ( S n o w , 1 9 7 2 ).

In o u r e x p e r i m e n t s , s a m p l e s u s e d f o r c o m p o s i t i o n a n a l y se s o f D a p h n i ao r i g i n a t e d f r o m a m i x t u r e o f v a r i o u s s i ze c a t e g o r i e s . I n t h e c a se o fs e l e c t iv e s a m p l i n g f a v o u r i n g l ar g e in d i v i d u a l s , t h e b i o m a s s e s h a r v e s t e dm a y c o n t a i n m o r e l i pi d s, o v i g e r o u s a d u l t s c o n t a i n i n g m o r e l ip id s o n ad r y w e i g h t b a s is t h a n j u v e n i l e s a n d p r e - a d u l t s ( I v le v a , 1 9 7 3 ) . O n th eo t h e r h a n d , c h a n g es in t h e c o m p o s i t i o n o f d a p h n i d b i o m a s s e s m a y b e


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106 D. Proulx, J. d e la No iieexpected as a function of microalgae quality which depends itself onthe quality of the secondary effluent (Lee and Picard, 1982). Thishypothesis should, however, be confirmed by further studies.

Using daphnid biomasses in fresh water aquaculture opens up inter-esting perspectives. As a matter of fact, daphnids are a major element ofthe diet of several groups of freshwater fish, such as the S a l m o n i d a e ,and may account at some periods of the year for more than 95-98% ofthe stomachic content of rainbow trout (Galbraith, 1967; Taylor andGerking, 1980). On the other hand, the gross composition of D a p h n i am a g n a grown on treated wastewaters as in our study, compares wellwith that of artificial diets for S a l rn o n i d a e and with that of A r t e m i asal ina , a zooplankton widely used in marine aquaculture (Table 1).

When live organisms are used for feeding fish, the specific needs ofthe latter in some components, such as amino acids and essential fattyacids, should be considered. In our study, the composition of daphnidsin amino acids differed (Table 2) from the values obtained for daphnidsgrown on rice bran (Morales, 1983) and for A r t e m i a s a l i n a (Gallagherand Brown, 1975), especially for lysine, histidine and arginine. Theamount of methionine in our daphnid and algae samples, which ishigher than the needs of Chinook salmon O n e o r h y n c h u s t s h a w y t s c h a ,is interesting since the deficiency in this amino acid is a characteristicof many live preys such as Bra c h i o n u s , T r i g r i o p u s (Gallagher andBrown, 1975) as well as D a p h n i a m a g n a grown on rice bran (Morales,1983). Moreover, the total essential amino acid contents of D a p h n i aand S c e n e d e s m u s grown on urban wastewaters is higher than the needsof Chinook salmon. Both these organisms could thus be promising foodsources for freshwater fish.

The lipid con ten t and composition of feed are also impor tant for thegrowth and reproduction of freshwater fish. According to Halver(1972), linoleic and linolenic acids (18: 3) are essential for the animalto synthesise long-chain polyunsaturated fatty acids. The fatty acidpatt ern obta ined in this study corresponds to that of Morales (1983)for populations of D . m a g n a grown on rice bran (Table 3). Ourdata on palmitoleic (C 16 : 1 ), oleic (C 18 : 1) and linoleic (C 18 : 2) acidcontents are lower than those of Morales; linolenic (C18:3) acid is,however, higher. This composition in fatty acids may vary as a functionof the fatty acid composition of microalgae, which depends itself onthe culture conditions (Klyachko-Gurvich, 1974). On the other hand,Morales (1983) has shown that daphnid biomasses may be enriched in


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Growing Daphnia magna on wastewaters 107s o m e f a t t y a c id s b y s u p p l e m e n t i n g th e d i e t o f d a p h n i d s w i t h c o d li ve ro i l, f o r e x a m p l e .

C O N C L U S I O NA s u s p e n s i o n o f t h e m i c r o a l g a e Scenedesmus s p . o r i g in a t i n g f ro m t h et e r ti a r y t r e a t m e n t o f w a s t e w a t e r s p r o v e d t o b e a n e x c e l l e n t p h y s i c a la n d n u t r i t i o n a l s u p p o r t f o r Daphnia. T h i s o b s e r v a t i o n i s a l l t h e m o r ei n t e re s t i n g s in c e w a s t e w a t e rs h a v e a h i g h c o n t e n t o f in o r g a n i c c o m -p o n e n t s , a re a b u n d a n t a n d r e p r e s e n t , v ia m i c r o a l g a e, a d i e t a r y s o u r c et h a t is a l m o s t i n e x h a u s t i b l e a n d u n e x p l o i t e d ( D i n g es , 1 9 7 4 ) f o r t h ep r o d u c t i o n o f l ar ge b i o m a s s e s o f a n i m a l o r ig in .

O u r r e su l ts o n t h e p r o d u c t i v i t y a n d e n e r g y c o n v e r s io n c o e f f i c ie n td e m o n s t r a t e t h e g r e a t p o t e n t i a l o f Daphnia f o r t h e p r o d u c t i o n o f a na n i m a l b i o m a s s t h a t c a n b e u s e d i n f r e s h w a t e r a q u a c u l t u r e . S i n c e t h ec o m p o s i t i o n o f d a p h n i d b i om a s s e s m a y v a ry a c c o rd i n g t o t h e q u a l it ya n d a v a il ab il it y o f f o o d ( L e m c k e a n d L a m p e r t , 1 9 7 5 ; M o r a le s , 1 9 8 3 ),f u r t h e r s t u d ie s a re n e e d e d o n t h e r e l a t io n s h i p s b e t w e e n t h e q u a l i t y o ft h e u r b a n s e c o n d a r y e f f lu e n t a n d t h e c o m p o s i t i o n o f d a p h n i d b io m a s se s .

F i na l ly , t h e c u l t u re o f d a p h n i d s i n a n o p e n s y s t e m h a s p r o v e d p ar ti -c u l a r ly p r o m i s i n g . O u r f u r t h e r r e s e a r c h w i ll a i m a t o p t i m i s i n g t h ee x p l o i t a ti o n o f Daphnia p o p u l a t i o n s o n w a s t ew a t e rs .

A C K N O W L E D G E M E N T ST h i s s t u d y w a s s u p p o r t e d b y t h e ' M i ni st 6 re d e l ' E d u c a t i o n d u Q u 6 b e c '( F C A C - t e a m g r a n t ) , t h e ' C o n s e i l d e r e c h e r c h e s en s c i e n c e s n a t u r e l l e s e te n g 6 n i e ' ( C R S N G ) , t h e 'C o n se i l d e s r e c h e r c h e s e t s e r v ic e s a g r i c o l e s d uQ u 6 b e c ' ( C R S A Q ) a n d IB M C a n a d a L t d . W e t h a n k M s C a r o le N oO l f o ra id a n d s u g g e s t i o n s i n p r e p a r i n g t h e m a n u s c r i p t .

T h e a u t h o r s a re g r a t e f u l t o M r J o y a l ( e n g i n e e r a t t h e w a s t e w a t e rt r e a t m e n t p l a n t o f V a l c a r t i e r' s m i l i t a r y b as e , Q u 6 b e c ) , M e s sr s J. D e b r o u x ,J . M a k h l o u f , C . G o sse l i n , P . W a l sh a n d T . P o u l i o t f o r t h e i r a s s i s t a n c e .

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